1. Field of the Invention
This invention relates to transmissions and particularly to multiple countershaft transmissions wherein means are provided to ensure a substantially equal distribution of torque on each countershaft.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Multiple countershaft transmissions have been used in heavy duty automotive applications as a way to transmit torque by using smaller gears than possible in a single countershaft transmission of equal input torque capacity. Design of and improvements to multiple countershaft transmissions are detailed in the following patents: U.S. Pat. Nos. 3,105,395; 3,237,472; 3,335,616; 3,500,695; 3,648,546; 3,799,002; 3,885,446; 3,924,484; 4,152,949; 4,640,145; 4,709,590; 4,807,493. These designs have common features to ensure substantially equal distribution of torque on each countershaft: (1) The countershaft assemblies require rigid, specific and precise rotational positioning of each countergear as installed on each countershaft. It is a typical requirement to maintain specified countergear tooth centerline rotational positioning accuracy within 0.001 inch to 0.002 inch at the gear tooth pitch line on each countergear with respect to the countershaft they are associated with. This level of precision can be difficult and costly to achieve and maintain in a mass production environment.
One method of manufacturing countershaft gear assemblies to this level of precision involves generating each countergear on an integral, common countershaft. Another method involves cutting a precisely located keyway in the bore of each countergear and pressing the countergears onto keyed countershafts. In some applications, engine torque levels have increased to the extent that this is no longer considered a reliable way to prevent relative motion between the countershaft and the countergears mounted thereon. Consequently, at least one manufacturer welds the installed countershaft gears to the countershaft or to adjacent countershaft gears to prevent this relative motion. Great care must be taken to ensure the welding process does not cause radial runout on each finished countershaft assembly. Such distortion can upset the radial and rotational positioning accuracy of each gear. In addition, if the countergear teeth are heat treated before the welding operation, care must be taken to ensure the welding process does not affect the heat treatment of the gear teeth. These measures add to the complexity and cost of the countershaft assembly manufacturing process.
Neither countershaft assembly manufacturing method allows for replacement of individual gears on the countershaft assembly. If one countershaft gear requires service replacement, the entire countershaft assembly must be replaced, significantly increasing repair costs.
Splined countershaft gear-to-countershaft connections are not used because the precise rotational gear positioning accuracy incumbent to this design is not attainable using such splined connections.
(2) Floating mainshaft driven gears and floating or non-floating mainshaft. Modern, multiple-countershaft transmissions typically have mainshaft driven gears which are free to radially float within narrow limits. The mainshaft may be free to radially float at one or both ends within narrow limits or the mainshaft may be of a non-floating design.
The first discussion will be of radially floating mainshaft driven gears and of a mainshaft that is free to radially float at one or both ends. With such a design, each driven mainshaft gear assumes a radial position to adjust for the inevitable manufacturing tolerances in the assembled transmission. Consequently, the location of the rotational axis of each driven mainshaft gear can independently vary. By design, the floating mainshaft aligns with the driven mainshaft gear as the selected mainshaft gear is mechanically coupled to the mainshaft during gear shifting. Aligning the significantly heavy mainshaft to the selected driven mainshaft gear during gear shifting has the negative effect of increasing gear shifting effort.
In addition, the axes of the selected driven mainshaft gear and the mainshaft are not necessarily parallel. This is because the driven mainshaft gears are constrained to move radially only, such that the rotational axis of each driven mainshaft gear remains substantially parallel to the rotational axis of each countershaft. The mainshaft, however, is allowed to move radially at one or both ends. Either freedom of movement of the mainshaft has the disadvantage of allowing the mainshaft to assume a non-parallel relationship to the selected driven mainshaft gear. This causes relative sliding motion between the mainshaft gear-to-mainshaft coupling elements as these elements rotate. This reduces the mechanical efficiency of the transmission and increases wear on these elements.
In addition, this non-parallel relationship can result in significant contact stress between the selected driven mainshaft gear and the mainshaft members used to limit the axial movement of the selected driven mainshaft gear on the mainshaft. This can become a greater disadvantage when helical gearing is used, since such gearing can generate significant axial thrust forces, thus increasing this contact stress and consequently increasing wear between these surfaces.
In addition, when the mainshaft is free to move radially at one or both ends, the mainshaft can have a nutating motion when the transmission is in neutral (when the mainshaft is not mechanically coupled to a driven mainshaft gear) and the mainshaft is rotating. If the mainshaft rotational speed is significant, undesirable vibration can result from the nutating motion. In addition, the vibration forces can create the disadvantage of further increasing shifting effort during gear changes.
The second discussion will be of radially floating driven mainshaft gears and a non-floating mainshaft. With such a design, each driven mainshaft gear assumes a radial position to adjust for the inevitable manufacturing tolerances in the assembled transmission. Consequently, the location of the rotational axis of each driven mainshaft gear can independently vary from the fixed mainshaft rotational axis. Such an arrangement has the disadvantage of radial sliding contact between the engaged driven mainshaft gear and the driven mainshaft member, reducing the mechanical efficiency of the transmission and increasing wear on these gear engagement surfaces.
In the first and second discussions, the driven mainshaft gears are permitted to radially float; therefore, the driven mainshaft gears do not necessarily engage their mating countershaft gears pitch line-to-pitch line; this can result in reduced mechanical efficiency and increased gear noise.
Since the prior art permits limited radial movement of mainshaft gears with respect to the mainshaft, it is impossible to employ standard synchromesh clutches for the various shifting operations, as such clutches require exact alignment of the various gears and shafts they act on. For this reason, more basic jaw-type clutches are used which allow limited radial displacement of the clutched parts relative to each other. The result of this type of construction is that shifting must be done with much care or with assist from a complex automated gear shifting mechanism in order to prevent grinding and clashing of gear clutch teeth.
U.S. Pat. No. 4,226,135 (1980) to Winter proposed a multiple countershaft transmission design which would allow the mainshaft gears to remain concentric to the mainshaft, consequently allowing the usage of synchromesh clutches. However, Winter's transmission is of a hybrid design, in that input torque is directed through only one countershaft in several of the transmission ratios and through two countershafts in other transmission ratios. In addition, as with other prior art, gear timing must be very precise (in at least some of the transmission ratios) to ensure substantially equal distribution of torque on each countershaft. In addition, Winter's design includes a relatively intricate mechanism to provide limited angular motion between countershaft gears and countershafts.